The present invention is concerned with radioelectric techniques and more particularly with links with satellites or other ballistic devices. The invention relates more specifically, on the one hand, to a synchronizer and demodulator for a message coded in biphase (and even B .phi. S), which modulates a subcarrier in phase, or, in other words, a PSK synchronizer and demodulator, and, on the other hand, to a device for eliminating ambiguity from a binary coded message, in the present case in N R Z, which is included in a synchronizer and demodulator of this kind.
So as not to leave room for any uncertainty as to the meaning of the conventional terms used above, as well as that of others which will appear in the course of this specification, ideas and definitions relating to these terms will be recited hereunder.
First of all, it is known that PSK (phase shift keying) telemetering is a method of modulating the phase of a subcarrier by a PCM signal, there being understood by subcarrier a wave carrying the message, this "subcarrier" modulating a "carrier" or not. Here the modulation considered is that imposed by a contingent message with two possible levels (+1 and -1), which is coded by a PCM, NRZ or biphase (B .phi.) signal (see FIG. 1). Consequently, PSK telemetering is characterised by the ratio n = f.sub.0 /F, f.sub.o being the subcarrier frequency and F the bit rate or rhythm.
PCM (Pulse Code Modulation) is a system multiplexed in time which is generally used when a large number of items of information must be transmitted in the form of successive pulses. Each pulse capable of assuming one of the two possible levels "0" and "1" is called a bit.
Each telemetering channel to be transmitted is binary coded to form one or more words. A group of a certain number of words constitutes a sequence or cycle and a group of several cycles constitutes a sub-cycle.
The PCM message transmitted is constituted by a group of words, cycles and sub-cycles which can be marked within the message by certain well-defined binary configurations.
Thus, the PCM message will appear as a sequence of 0's and 1's of a contingent nature transmitted in accordance with a code to be chosen, usually from among those given hereunder. (The duration of a period of the code will be called T; the digital rate or rhythm is F =0 1/T).
1 -- NRZ (no-return-to-zero) Codes The entire period constitutes the support for the information.
A. NRZ-L (Level) or NRZ-C (Change) Code A 1 or a 0 is represented by a high level or by a low level (according to the convention adopted). There is therefore a transition every time the bit changes state (from 0 to 1 or from 1 to 0). PA1 b. NRZ-M (Mark) Code The appearance of a 1 is marked by a transition of level, positive or negative according to whether the preceding level was low or high; the 0's have no effect. PA1 c. NRZ-S (Space) Code The appearance of a 0 is marked by a transition of the level, positive or negative according to the preceding level; the 1's have no effect. PA1 a. Biphase L or C (B .phi.-L or B .phi.-C) Code The information is represented by a transition in the middle of the period: negative transition for a 1, positive transition for a 0, for example. PA1 b. Biphase S (B .phi.-S) Code This is elaborated from the NRZ-M code; the appearance of a 1 is manifested by an inversion in the direction of the transition delayed by a half-period. PA1 c. Biphase M (B .phi.-M) Code This is elaborated from the NRZ-S code; the appearance of a O is manifested by an inversion in the direction of the transition delayed by a half-period. The B .phi.-S code is elaborated from the NRZ-M code.
2 -- Biphase Codes Biphase codes are the result of the multiplication of the NRZ codes by the digital rate F in phase with the signal.
The above-defined codes are illustrated in FIG. 2 of the accompanying drawings.
These definitions having been given, it is furthermore appropriate to state precisely that the invention relates to a PSK synchronizer and demodulator designed to receive a PSK transmission constituted by a subcarrier modulated in Biphase S and serving to produce a coherent demodulation of the message and to extract therefrom a local clock signal and a message in NRZ code (and even NRZ-C), it being possible for the lastmentioned message to be decoded by using the local clock signal created in this way, the whole in the presence of noise.
After extraction of the message and its demodulation, a PSK synchronizing and demodulating apparatus generally supplies information still having a certain ambiguity, that is to say it is not certain whether the 1's are not in place of the 0's and vice versa, this being due to a similar uncertainty concerning the restoration of the locally recreated clock rhythm F, and after division by 2 or a power of 2. Consequently, a synchronizing and demodulating apparatus of this kind generally comprises a device intervening at the end of the demodulation operation and eliminating the ambiguity in question.
One of the methods which is used most to this end consists in detecting the biphase signal and in carrying out, from this signal, a statistical count of these transitions at mid-bit and at the beginning of a bit. According to the resulting value of the calculating counter (maximum or zero value), it is decided either to retain or to invert the clock signal F.
Another method consists in filtering the NRZ signal and, starting from this signal and the assumed transitions, in triggering a monostable which will put the divider or the dividing chain back into phase.
These methods are based on the detection of the transitions of the message drowned in the noise and they present the disadvantage of having a considerable threshold, beyond which recognition of the transitions of the message cannot be effected correctly, these transitions being merged in the noise.
Orders for putting the signal having the clock rhythm F back into phase therefore appear at the wrong time and thereby render the information detected unexploitable.